The present invention relates generally to a condenser and illumination systems for projecting the image of a reticle onto a photosensitive substrate as used in photolithography in semiconductor manufacturing, and more particularly to a condenser suitable for use in the extreme ultraviolet or soft X-ray wavelengths having non-imaging optics forming a desired irradiance and desired angular distribution.
Photolithography is often used in the manufacture of many devices and in particular, electronic or semiconductor devices. In a photolithographic process, the image of a reticle or mask is projected onto a photosensitive substrate. As the element or feature size desired to be imaged on the photosensitive substrate becomes ever smaller, technical problems often arise. One of these problems is illuminating the reticle or mask so that its image can be projected onto the photosensitive substrate. As the element or feature size of semiconductor devices become ever smaller, there is a need for photolithographic systems providing a resolution of less than 0.13 micrometers. In order to achieve the imaging of these relatively small element or feature sizes, shorter wavelengths of electromagnetic radiation must be used to project the image of a reticle or mask onto the photosensitive substrate. Accordingly, it is necessary for photolithographic systems to operate at the extreme ultraviolet wavelengths, below 157 nanometers, and into the soft X-ray wavelengths, around 1 nanometers. Additionally, projection optics having the required resolution and imaging capabilities often result in utilization of a portion of a ring field. One such projection optic system used in photolithography is disclosed in U.S. Pat. No. 5,815,310 entitled xe2x80x9cHigh Numerical Aperture Ring Field Optical Reduction Systemxe2x80x9d issuing to Williamson on Sep. 29, 1998, which is herein incorporated by reference in its entirety. While the projection optic system disclosed therein can achieve a working resolution of 0.03 microns, there are few illumination sources or illumination systems that can provide the required illumination properties for projecting the image of the reticle or mask onto the photosensitive substrate. An illuminating system is disclosed in U.S. Pat. No. 5,339,346 entitled xe2x80x9cDevice Fabrication Entailing Plasma-Derived X-Ray Delineationxe2x80x9d issuing to White on Aug. 16, 1994. Therein disclosed is a condenser for use with a laser-pumped plasma source having a faceted collector lens including paired facets, symmetrically placed about an axis. Another illumination system is disclosed in U.S. Pat. No. 5,677,939 entitled xe2x80x9cIlluminating Apparatusxe2x80x9d issuing to Oshino on Oct. 14, 1997. Therein disclosed is an illumination system for illuminating an object in an arcuate pattern having a reflecting mirror with a parabolic-toric body of rotation in the reflection type optical integrator having a reflecting surface for effecting the critical illumination in the meridoinal direction and a reflecting surface for effecting the Kohler illumination in the sagittal direction. Another illuminating system is disclosed in U.S. Pat. No. 5,512,759 entitled xe2x80x9cCondenser For Illuminating A Ring Field Camera With Synchrotron Emission Lightxe2x80x9d issuing to Sweatt on Apr. 30, 1996, which is herein incorporated by reference in its entirety. Therein disclosed is a condenser comprising concave and convex spherical mirrors that collect the light beams, flat mirrors that converge and direct the light beams into a real entrance pupil of a camera, and a spherical mirror for imaging the real entrance pupil through the resistive. mask and into the virtual entrance pupil of the camera. Another illumination system is disclosed in U.S. Pat. No. 5,361,292 entitled xe2x80x9cCondenser For Illuminating A Ring Fieldxe2x80x9d issuing to Sweatt on Nov. 1, 1994. Therein disclosed is a condenser using a segmented aspheric mirror to collect radiation and produce a set of arcuate foci that are then translated and rotated by other mirrors so that all the arcuate regions are superposed at the mask.
However, these prior illumination systems may not provide the desired illumination and are relatively complicated. Additionally, many of these systems are relatively large, having many surfaces resulting in loss of energy. Some are also difficult to align and may require adjustment.
Accordingly, there is a need for an improved illumination system and condenser for use in the extreme ultraviolet that provides a desired irradiance over a predetermined field or area with a desired irradiance and angular distribution for use in photolithography.
The present invention is directed to an illumination system comprising a condenser having non-imaging optic elements. A first non-imaging optic element is used to collect light from a source and create a desired or predetermined irradiance distribution. A second non-imaging optic element receives electromagnetic radiation from the first non-imaging optic element and redirects the electromagnetic radiation into collimated nearly spherical or flat wavefronts. Facets placed at the pupil of the illumination system shapes the electromagnetic radiation and provides uniform illumination over a desired area. The facets may be provided on the second non-imaging optic element. Additional objective optics may be utilized to further-process the electromagnetic radiation or to relay the electromagnetic radiation to the desired area at a reticle or mask, the image of which is projected onto a photosensitive substrate.
Accordingly, it is an object of the present invention to provide a desired irradiance over a predetermined field or area.
It is a further object of the present invention to provide a predetermined angular and irradiance distribution.
It is yet a further object of the present invention to increase the xc3xa9tendue of a source of electromagnetic radiation.
It is an advantage of the present invention that it is an efficient condenser for the desired wavelength.
It is a further advantage of the present invention that it is relatively compact.
It is a feature of the present invention that non-imaging optic elements are used.
It is another feature of the present invention that a relatively small number of reflective surfaces are utilized.
It is yet a further feature of the present invention that a faceted optic element is used.
These and other objects, advantages, and features will be readily apparent in view of the following detailed description.